Like a sensor network, there exists a system in which a large number of clients installed in a various environment collect information and upload information in a server for information collection via a network. Such a system can grasp information of a remote location from a client which is installed in the environment via a network. Also, by using such a system, it is possible to improve operation efficiency compared with a method in which a person patrols each location and manages it.
For example, an example of a sensor network is disclosed in Patent document 1. FIG. 14 is an explanatory drawing showing an example of the sensor network disclosed in Patent document 1. As shown in FIG. 14, a sensor network 900 disclosed in Patent document 1 includes a plurality of sensor devices (client) 901 . . . equipped with a wireless communication function and external applications (server) 921, 922, 923 . . . .
A purpose of use of the sensor network 900 is diverse. The sensor network 900 can be used for all the fields such as crime prevention, disaster prevention, nursing, energy conservation, wide area measurement and tracing. For example, in the field of nursing, the sensor network 900 can monitor using a sensor device 901 heart rate, body temperature, posture, movement of hand and foot, emergency and so on. Here, the sensor device 901 often does not hold high processing power in the device itself because of problems for its production cost and electric power consumption.
A sensor network including a structure mentioned above operates as follows. That is, in the sensor network, when a server collects data from a large number of clients installed in a wide range in the environment, the server establishes a path to the clients by wireless communication, and the clients transmit information via the established wireless communication path.
In an operation of this sensor network, a network key is shared by all the clients belonging to the network, and appropriateness to participate in the network is often judged based on whether the network key is possessed or not. In such an operation, in case necessity occurs to disconnect a specific client from the network, disabling the network key which the client possesses will become a problem. A network key update method related with such a problem is disclosed in Non-patent document 1. A related method 1 disclosed in Non-patent document 1 encrypts in advance, between the server and an arbitrary client, a network key for each client using a key shared one to one (terminal unique key) and distributes the encrypted network key to each terminal separately.
Also a related method 2 disclosed in Non-patent document 2 reduces a number of communications used for network key distribution using a tree structure called a key tree. In the method, a key is allocated to each vertex of the key tree.
In the method, a leaf and a client correspond one to one, and a key is defined for each parent vertex in a bottom-up way from leaves. Also, the client possesses all the keys of its ancestor. At this time, because all the clients share a key of a root of the key tree, the key of the root is used as a network key.
In this method, when a client is disconnected, the keys which the disconnected node holds are all updated. At this time, this method selects a vertex among vertices other than those become the ancestor of the client to be disconnected and from vertices near the root of the key tree in top-down way; and divides the key tree into subtrees which have each selected vertex as a root.
This method aims at efficient realization of key update by sending information in a simultaneous forwarding way to each subtree divided using a key which is allocated to the root.
Further, it is often assumed that information stored in a server and a client secretly is managed safely by an operation such as using a tamper resistant device (that is, a device which is very difficult to leak the internal classified information unlawfully) disclosed, for example, in Patent document 2. In the following, in case there is a description to store secretly, it is supposed that the information is stored secretly under the assumption that unlawful leak is very difficult by such the operation.
Also, a related technology is disclosed in Patent document 3. The technology disclosed in Patent document 3 is a method which, by structuring a key shared between a server and a node appropriately, enables multicasting an encrypted simultaneous forwarding communication message to be performed efficiently.
Also, a related technology is disclosed in Patent document 4. The technology disclosed in Patent document 4 uses an encryption key. And the technology is a mechanism which, in a network communication which communicates using a plurality of communication apparatus, and being prepared for a case when a structure of a group in the communication apparatus changes according to a progress of time, updates a group key which is an encryption key, and distributes the key after update safely in the group.